It is essential for establishing and maintaining pregnancy in females.
In the ovary, the corpus luteum secretes estrogens and progesterone, which are steroid hormones responsible for the thickening of the endometrium and its development and maintenance, respectively. If the egg is not fertilized, the corpus luteum stops secreting progesterone and decays (after approximately 14 days in humans). It then degenerates into a corpus albicans, which is a mass of fibrous scar tissue. The uterine lining sloughs off without progesterone and is expelled through the vagina (in humans and some great apes).In an estrus cycle the lining degenerates back to normal size.

If fertilized, however, the placenta secretes the hormone human chorionic gonadotropin (hCG) or a similar hormone in other species. This hormone signals the corpus luteum to continue progesterone secretion, thereby maintaining the thick lining (endometrium) of the uterus, and providing an area rich in blood vessels in which the zygote(s) can develop. From this point on, the corpus luteum is called the corpus luteum graviditatis. The introduction of the hormone prostaglandin at this point causes the degeneration of the corpus luteum and the abortion of the fetus.

The corpus luteum is typically very large relative to the size of the ovary; in humans, the size of the structure ranges from under 2 cm to 6 cm in diameter. [1] Its cells develop from the follicular cells surrounding the ovarian follicle. The granulosa cells become the outer granulosa lutein layer and secrete progesterone. Theca interna cells become the inner theca lutein layer which secretes estrogen.